The mechanism of the antigenic variation shown by the relapsing fever borreliae is under study. Having identified serotype-specific, molecular weight-variable proteins at the surface of Borrelia hermsii, we analyzed the structure of these serotype-specific proteins (SSP) by peptide mapping procedures and immunochemical techniques. The SSPs differed considerably in their peptide maps and in their reactions with monoclonal antibodies and polyclonal antisera. This apparent paucity of amino acid sequence homology suggests possible models for the mechanism underlying SSP variability. In our continuing investigations of the genetic mechanism of borrelial antigenic variation, we characterized the genome of B. hermsii and identified extrachromosomal elements in the HS1 strain.